I. The Hematopoietic System
The mammalian hematopoietic system produces a large number of mature highly specialized blood cells. These mature cells include: erythrocytes specialized to transport oxygen and carbon dioxide, T and B lymphocytes responsible for cell- and antibody-mediated immune responses, platelets or thrombocytes specialized to form blood clots, and granulocytes and macrophages specialized as scavengers and as accessory cells to combat infection. Granulocytes are further subdivided into; neutrophils, eosinophils, basophils and mast cells, specialized cell types having discrete functions. Remarkably, these specialized mature blood cells are all derived from a single common primitive cell type, referred to as the pluripotential (totipotent) stem cells, found primarily in bone marrow (Dexter et al., Ann. Rev. Cell Biol., 3:423-441 [1987]).
Large numbers of mature blood cells are produced continuously throughout life, the vast majority of which are destined to remain functionally active for only a few hours to weeks (Cronkite et al., Blood Cells, 2:263-284 [1976]). Thus, continuous renewal of mature blood cells, the primitive stem cells themselves, as well as any intermediate or lineage-committed progenitor cell lines is necessary to maintain the normal steady state blood cell needs of the mammal.
At the heart of the hematopoietic system is the pluripotent stem cell(s). These are relatively few in number and undergo self-renewal by proliferation to produce daughter stem cells or are transformed, in a series of differentiation steps, into increasingly mature lineage-restricted progenitor cells.
For example, certain multipotent progenitor cells, referred to as CFC-Mix, derived from stem cells undergo proliferation (self-renewal) and development to produce colonies containing all the different myeloid cells: erythrocytes, neutrophils, megakaryocytes (predecessors of platelets), macrophages, basophils, eosinophils, and mast cells. Other progenitor cells of the lymphoid lineage undergo proliferation and development into T-cells and B-cells.
Additionally, between the CFC-Mix progenitor cells and myeloid cells lie another rank of progenitor cells of intermediate commitment to their progeny. These lineage-restricted progenitor cells are classified on the basis of the progeny they produce. Thus, the known immediate predecessors of the myeloid cells are: erythroid colony-forming units (CFU-E) for erythrocytes, granulocyte/macrophage colony-forming cells (GM-CFC) for neutrophils and macrophages, megakaryocyte colony-forming cells (Meg-CFC) for megakaryocytes, eosinophil colony-forming cells (Eos-CFC) for eosinophils, and basophil colony-forming cells (Bas-CFC) for mast cells. Other intermediate predecessor cells between the pluripotent stem cells and mature blood cells are known (see below) or will likely be discovered having varying degrees of lineage-restriction and self-renewal capacity.
The underlying principal of the normal hematopoietic cell system appears to be decreased capacity of self-renewal as multipotency is lost and lineage-restriction and maturity is acquired. Thus, at one end of the hematopoietic cell spectrum lies the pluripotent stem cell possessing the capacity for self-renewal and differentiation into various lineage-specific committed progenitor cells. This capacity is the basis of bone marrow transplant therapy where stem cells reconstitute the entire hematopoietic cell system. At the other end of the spectrum lie the highly lineage-restricted progenitors and their progeny which have lost the ability of self-renewal but have acquired mature functional activity.
The proliferation and development of stem cells and lineage-restricted progenitor cells is carefully controlled by a variety of hematopoietic growth factors or cytokines. The role of these growth factors in vivo is complex and incompletely understood. Some growth factors, such as interleukin-3 (IL-3), are capable of stimulating both multipotent stem cells as well as committed progenitor cells of several lineages, including for example, megakaryocytes. Other factors such as granulocyte/macrophage colony-stimulating factor (GM-CSF) was initially thought to be restricted in its action to GM-CFC's. Later, however, it was discovered GM-CSF also influenced the proliferation and development of interalia megakaryocytes. Thus, IL-3 and GM-CSF were found to have overlapping biological activities, although with differing potency. More recently, both interleukin-6 (IL-6) and interleukin-11 (IL-11), while having no apparent influence on meg-colony formation alone, act synergistically with IL-3 to stimulate maturation of megakaryocytes (Yonemura et al., Exp. Hematol., 20:1011-1016 [1992]).
Thus, hematopoietic growth factors may influence growth and differentiation of one or more lineages, may overlap with other growth factors in affecting a single progenitor cell line, or may act synergistically with other factors.
It also appears that hematopoietic growth factors can exhibit their effect at different stages of cell development from the totipotent stem cell through various committed lineage-restricted progenitors to the mature blood cell. For example, erythropoietin (EPO) appears to promote proliferation only of mature erythroid progenitor cells. IL-3 appears to exert its effect earlier influencing primitive stem cells and intermediate lineage-restricted progenitor cells. Other growth factors such as stem cell factor (SCF) may influence even more primitive cell development.
It will be appreciated from the foregoing that novel hematopoietic growth factors that affect survival, proliferation, differentiation or maturation of any of the blood cells or predecessors thereof would be useful, especially to assist in the re-establishment of a diminished hematopoietic system caused by disease or after radiation- or chemotherapy.
II. Megakaryocytopoiesis
Regulation of megakaryocytopoiesis and platelet production has been reviewed by: Mazur, E. M., Exp. Hematol. 15:248 [1987] and Hoffman, R., Blood, 74:1196-1212 [1989]. Briefly, bone marrow pluripotent stem cells differentiate into megakaryocytic, erythrocytic, and myelocytic cell lines. It is believed there is a hierarchy of committed megakaryocytic progenitor cells between stem cells and megakaryocytes. At least three classes of megakaryocytic progenitor cells have been identified, namely; burst forming unit megakaryocytes (BFU-MK), colony-forming unit megakaryocytes (CFU-MK), and light density megakaryocyte progenitor cells (LD-CFU-MK). Megakaryocytic maturation itself is a continuum of development that has been separated into stages based on standard morphologic criteria. The earliest recognizable member of the megakaryocyte (MK) family are the megakaryoblasts. These cells are initially 20 to 30 μm in diameter having basophilic cytoplasm and a slightly irregular nucleus with loose, somewhat reticular chromatin and several nucleoli. Later, megakaryoblasts may contain up to 32 nuclei, but the cytoplasm remains sparse and immature. As maturation proceeds, the nucleus becomes more lobulate and pyknotic, the cytoplasm increases in quantity and becomes more acidophilic and granular. The most mature cells of this family may give the appearance of releasing platelets at their periphery. Normally, less than 10% of megakaryocytes are in the blast stage and more than 50% are mature. Arbitrary morphologic classifications commonly applied to the megakaryocyte series are megakaryoblast for the earliest form; promegakaryocyte or basophilic megakaryocyte for the intermediate form; and mature (acidophilic, granular, or platelet-producing) megakaryocyte for the late forms. The mature megakaryocyte extends filaments of cytoplasm into sinusoidal spaces where they detach and fragment into individual platelets (Williams et al., Hematology, 1972).
Megakaryocytopoiesis is believed to involve several regulatory factors (Williams et al., Br. J. Haematol., 52:173 [1982] and Williams et al., J. Cell Physiol., 110:101 [1982]). The early level of megakaryocytopoiesis is postulated as being mitotic, concerned with cell proliferation and colony initiation from CFU-MK but is not affected by platelet count (Burstein et al., J. Cell Physiol., 109:333 [1981] and Kimura et al., Exp. Hematol., 13:1048 [1985]). The later stage of maturation is non-mitotic, involved with nuclear polyploidization and cytoplasmic maturation and is probably regulated in a feedback mechanism by peripheral platelet number (Odell et al., Blood, 48:765 [1976] and Ebbe et al., Blood, 32:787 [1968]). The existence of a distinct and specific megakaryocyte colony-stimulating factor (MK-CSF) has been disputed (Mazur, E., Exp. Hematol., 15:340-350 [1987]). However most authors believe that a process so vital to survival as platelet production would be regulated by cytokine(s) exclusively responsible for this process. The hypothesis that megakaryocyte/platelet specific cytokine(s) exist has provided the basis for more than 30 years of search but to date no such cytokine has been purified, sequenced and established by assay as a unique MK-CSF.
Although it has been reported that MK-CSF's have been partly purified from experimentally produced thrombocytopenia (Hill et al., Exp. Hematol., 14:752 [1986]) and human embryonic kidney conditioned medium [CM] (McDonald et al., J. Lab. Clin. Med., 85:59 [1975]) and in man from a plastic anemia and idiopathic thrombocytopenic purpura urinary extracts (Kawakita et al., Blood, 6:556 [1983]) and plasma (Hoffman et al., J. Clin. Invest., 75:1174 [1985]), their physiological function is as yet unknown in most cases.
The conditioned medium of pokeweed mitogen-activated spleen cells (PWM-SpCM) and the murine myelomonocyte cell line WEHI-3 (WEHI-3CM) have been used as megakaryocyte potentiators. PWM-SpCM contains factors enhancing CFU-MK growth (Metcalf et al., Pro. Natl. Acad. Sci., USA, 72:1744-1748 [1975]; Quesenberry et al., Blood, 65:214 [1985]; and Iscove, N. N., in Hematopoietic Cell Differentiation, ICN-UCLA Symposia on Molecular and Cellular Biology, Vol. 10, Golde et al., eds. [New York, Academy Press] pp 37-52 [1978]), one of which is interleukin-3 (IL-3), a multilineage colony stimulating factor (multi-CSF [Burstein, S. A., Blood Cells, 11:469 [1986]). The other factors in this medium have not yet been identified and isolated. WEHI-3 is a murine myelomonocytic cell line secreting relatively large amounts of IL-3 and smaller amounts of GM-CSF. IL-3 has been found to potentiate the growth of a wide range of hematopoietic cells (Ihle et al., J. Immunol., 13:282 [1983]). IL-3 has also been found to synergize with many of the known hematopoietic hormones or growth factors (Bartelmez et al., J. Cell Physiol., 122:362-369 [1985] and Warren et al., Cell, 46:667-674 [1988]), including both erythropoietin (EPO) and interleukin-1 (IL-1), in the induction of very early multipotential precursors and the formation of very large mixed hematopoietic colonies.
Other sources of megakaryocyte potentiators have been found in the conditioned media of murine lung, bone, macrophage cell lines, peritoneal exudate cells and human embryonic kidney cells. Despite certain conflicting data (Mazur, E., Exp. Hematol., 15:340-350 [1987]), there is some evidence (Geissler et al., Br. J. Haematol., 60:233-238 [1985]) that activated T lymphocytes rather than monocytes play an enhancing role in megakaryocytopoiesis. These findings suggest that activated T-lymphocyte secretions such as interleukins may be regulatory factors in MK development (Geissler et al., Exp. Hematol., 15:845-853 [1987]). A number of studies on megakaryocytopoiesis with purified erythropoietin EPO (Vainchenker et al., Blood, 54:940 [1979]; McLeod et al., Nature, 261:492-4 [1976]; and Williams et al., Exp. Hematol., 12:734 [1984]) indicate that this hormone has an enhancing effect on MK colony formation. This has also been demonstrated in both serum-free and serum-containing cultures and in the absence of accessory cells (Williams et al., Exp. Hematol., 12:734 [1984]). EPO was postulated to be involved more in the single and two-cell stage aspects of megakaryocytopoiesis as opposed to the effect of PWM-SpCM which was involved in the four-cell stage of megakaryocyte development. The interaction of all these factors on both early and late phases of megakaryocyte development remains to be elucidated.
Data produced from several laboratories suggests that the only multi-lineage factors that individually have MK-colony stimulating activity are GM-CSF and IL-3 and, to a lesser extent, the B-cell stimulating factor IL-6 (Ikebuchi et al., Proc. Natl. Acad. Sci. USA, 84:9035 [1987]). More recently, several authors have reported that IL-11 and leukemia inhibitory factor (LIF) act synergistically with IL-3 to increase megakaryocyte size and ploidy (Yonemura et al., British Journal of Hematology, 84:16-23 [1993]; Burstein et al., J. Cell. Physiol., 153:305-312 [1992]; Metcalf et al., Blood, 76:50-56 [1990]; Metcalf et al., Blood, 77:2150-2153 [1991]; Bruno et al., Exp. Hematol., 19:378-381 [1991]; and Yonemura et al., Exp. Hematol., 20:1011-1016 [1992]).
Other documents of interest include: Eppstein et al., U.S. Pat. No. 4,962,091; Chong, U.S. Pat. No. 4,879,111; Fernandes et al., U.S. Pat. No. 4,604,377; Wissler et al., U.S. Pat. No. 4,512,971; Gottlieb, U.S. Pat. No. 4,468,379; Bennett et al., U.S. Pat. No. 5,215,895; Kogan et al., U.S. Pat. No. 5,250,732; Kimura et al., Eur. J. Immunol., 20(9):1927-1931 [1990]; Secor et al., J. of Immunol., 144(4):1484-1489 [1990]; Warren et al., J. of Immunol., 140(1):94-99 [1988]; Warren et al., Exp. Hematol., 17(11):1095-1099 [1989]; Bruno et al., Exp. Hematol., 17(10):1038-1043 [1989]; Tanikawa et al., Exp. Hematol., 17(8):883-888 [1989]; Koike et al., Blood, 75(12):2286-2291 [1990]; Lotem, Blood, 75(5):1545-1551 [1989]; Rennick et al., Blood, 73(7):1828-1835 [1989]; and Clutterbuck et al., Blood, 73(6):1504-1512 [1989].
III. Thrombocytopenia
Platelets are critical elements of the blood clotting mechanism. Depletion of the circulating level of platelets, called thrombocytopenia, occurs in various clinical conditions and disorders. Thrombocytopenia is commonly defined as a platelet count below 150×109 per liter. The major causes of thrombocytopenia can be broadly divided into three categories on the basis of platelet life span, namely; (1) impaired production of platelets by the bone marrow, (2) platelet sequestration in the spleen (splenomegaly), or (3) increased destruction of platelets in the peripheral circulation (e.g., autoimmune thrombocytopenia or chemo- and radiation-therapy). Additionally, in patients receiving large volumes of rapidly administered platelet-poor blood products, thrombocytopenia may develop due to dilution.
The clinical bleeding manifestations of thrombocytopenia depend on the severity of thrombocytopenia, its cause, and possible associated coagulation defects. In general, patients with platelet counts between 20 and 100×109 per liter are at risk of excessive post traumatic bleeding, while those with platelet counts below 20×109 per liter may bleed spontaneously. These latter patients are candidates for platelet transfusion with attendant immune and viral risk. For any given degree of thrombocytopenia, bleeding tends to be more severe when the cause is decreased production rather than increased destruction of platelets; in the latter situation, accelerated platelet turnover results in the circulation of younger, larger and hemostatically more effective platelets. Thrombocytopenia may result from a variety of disorders briefly described below. A more detailed description may be found in Schafner, A. I., “Thrombocytopenia and Disorders of Platelet Function,” Internal Medicine, 3rd Ed., John J. Hutton et al., Eds., Little Brown and Co., Boston/Toronto/London [1990].
(a) Thrombocytopenia Due to Impaired Platelet Production
Causes of congenital thrombocytopenia include constitutional aplastic anemia (Fanconi syndrome) and congenital amegakaryocytic thrombocytopenia, which may be associated with skeletal malformations. Acquired disorders of platelet production are caused by either hypoplasia of megakaryocytes or ineffective thrombopoiesis. Megakaryocytic hypoplasia can result from a variety of conditions, including marrow aplasia (including idiopathic forms or myelosuppression by chemotherapeutic agents or radiation therapy), myelfibrosis, leukemia, and invasion of the bone marrow by metastatic tumor or granulomas. In some situations, toxins, infectious agents, or drugs may interfere with thrombopoiesis relatively selectively; examples include transient thrombocytopenias caused by alcohol and certain viral infections and mild thrombocytopenia associated with the administration of thiazide diuretics. Finally, ineffective thrombopoiesis secondary to megaloblastic processes (folate or B12 deficiency) can also cause thrombocytopenia, usually with coexisting anemia and leukopenia.
Current treatment of thrombocytopenias due to decreased platelet production depends on identification and reversal of the underlying cause of the bone marrow failure. Platelet transfusions are usually reserved for patients with serious bleeding complications, or for coverage during surgical procedures, since isoimmunization may lead to refractoriness to further platelet transfusions. Mucosal bleeding resulting from severe thrombocytopenia may be ameliorated by the oral or intravenous administration of the antifibrinolytic agents. Thrombotic complications may develop, however, if antifibrinolytic agents are used in patients with disseminated intravascular coagulation (DIC).
(b) Thrombocytopenia Due to Splenic Sequestration
Splenomegaly due to any cause may be associated with mild to moderate thrombocytopenia. This is a largely passive process (hypersplenism) of splenic platelet sequestration, in contrast to the active destruction of platelets by the spleen in cases of immunomediated thrombocytopenia discussed below. Although the most common cause of hypersplenism is congestive splenomegaly from portal hypertension due to alcoholic cirrhosis, other forms of congestive, infiltrative, or lymphoproliferative splenomegaly are also associated with thrombocytopenia. Platelet counts generally do not fall below 50×109 per liter as a result of hypersplenism alone.
(c) Thrombocytopenia Due to Nonimmune-Mediated Platelet Destruction
Thrombocytopenia can result from the accelerated destruction of platelets by various nonimmunologic processes. Disorders of this type include disseminated intravascular coagulation, prosthetic intravascular devices, extra corporeal circulation of the blood, and thrombotic microangiopathies such as thrombotic thrombocytic purpura. In all of these situations, circulating platelets that are exposed to either artificial surfaces or abnormal vascular intima either are consumed at these sites or are damaged and then prematurely cleared by the reticuloendothelial system. Disease states or disorders in which disseminated intravascular coagulation (DIC) may arise are set forth in greater detail in Braunwald et al. (eds), Harrison's Principles of Internal Medicine, 11th Ed., p. 1478, McGraw Hill [1987]. Intravascular prosthetic devices, including cardiac valves and intra-aortic balloons can cause a mild to moderate destructive thrombocytopenia and transient thrombocytopenia in patients undergoing cardiopulmonary bypass or hemodialysis may result from consumption or damage of platelets in the extra corporeal circuit.
(d) Drug-Induced Immune Thrombocytopenia
More than 100 drugs have been implicated in immunologically mediated thrombocytopenia. However, only quinidine, quinine, gold, sulfonamides, cephalothin, and heparin have been well characterized. Drug-induced thrombocytopenia is frequently very severe and typically occurs precipitously within days while patients are taking the sensitizing medication.
(e) Immune (Autoimmune) Thrombocytopenic Purpura (ITP)
ITP in adults is a chronic disease characterized by autoimmune platelet destruction. The autoantibody is usually IgG although other immunoglobulins have also been reported. Although the autoantibody of ITP has been found to be associated with platelet membrane GPIIbIIIa, the platelet antigen specificity has not been identified in most cases. Extravascular destruction of sensitized platelets occurs in the reticuloendothelial system of the spleen and liver. Although over one-half of all cases of ITP are idiopathic, many patients have underlying rheumatic or autoimmune diseases (e.g., systemic lupus erythematosus) or lymphoproliferative disorders (e.g., chronic lymphocytic leukemia).
(f) HIV-Induced ITP
ITP is an increasingly common complication of HIV infection (Morris et al., Ann. Intern. Med., 96:714-717 [1982]), and can occur at any stage of the disease progression, both in patients diagnosed with the Acquired Immune Deficiency Syndrome (AIDS), those with AIDS-related complex, and those with HIV infection but without AIDS symptoms. HIV infection is a transmissible disease ultimately characterized by a profound deficiency of cellular immune function as well as the occurrence of opportunistic infection and malignancy. The primary immunologic abnormality resulting from infection by HIV is the progressive depletion and functional impairment of T lymphocytes expressing the CD4 cell surface glycoprotein (Lane et al., Ann. Rev. Immunol., 3:477 [1985]). The loss of CD4 helper/inducer T cell function probably underlies the profound defects in cellular and humoral immunity leading to the opportunistic infections and malignancies characteristic of AIDS (H. Lane supra).
Although the mechanism of HIV-associated ITP is unknown, it is believed to be different from the mechanism of ITP not associated with HIV infection. (Walsh et al., N. Eng. J. Med., 311:635-639 [1984]; and Ratner, L., Am. J. Med., 86:194-198 [1989]).
IV. Therapy
The therapeutic approach to the treatment of patients with HIV-induced ITP is dictated by the severity and urgency of the clinical situation. The treatment is similar for HIV-associated and non-HIV-related ITP, and although a number of different therapeutic approaches have been used, the therapy remains controversial.
Platelet counts in patients diagnosed with ITP have been successfully increased by glucocorticoid (e.g., prednisolone) therapy, however in most patients, the response is incomplete, or relapse occurs when the glucocorticoid dose is reduced or its administration is discontinued. Based upon studies with patients having HIV-associated ITP, some investigators have suggested that glucocorticoid therapy may result in predisposition to AIDS. Glucocorticoids are usually administered if platelet count falls below 20×109/liter or when spontaneous bleeding occurs.
For patients refractory to glucocorticoids, the compound 4-(2-chlorphenyl)-9-methyl-2-[3-(4-morpholinyl)-3-propanon-1-yl]6H-thieno[3,2,f][1,2,4]triazolo[4,3,a,][1,4]diazepin (WEB 2086) has been successfully used to treat a severe case of non HIV-associated ITP. A patient having platelet counts of 37,000-58,000/μl was treated with WEB 2086 and after 1-2 weeks treatment platelet counts increased to 140,000-190,000/μl. (EP 361,077 and Lohman et al., Lancet, 1147 [1988]).
Although the optimal treatment for acquired amegakaryocytic thrombocytopenia purpura (AATP) is uncertain, antithymocyte globulin (ATG), a horse antiserum to human thymus tissue, has been shown to produce prolonged complete remission (Trimble et al., Am. J. Hematol., 37:126-127 [1991]). A recent report however, indicates that the hematopoietic effects of ATG are attributable to thimerosal, where presumably the protein acts as a mercury carrier (Panella et al., Cancer Research, 50:4429-4435 [1990]).
Good results have been reported with splenectomy. Splenectomy removes the major site of platelet destruction and a major source of autoantibody production in many patients. This procedure results in prolonged treatment-free remissions in a large number of patients. However, since surgical procedures are generally to be avoided in immune compromised patients, splenectomy is recommended only in severe cases of HIV-associated ITP, in patients who fail to respond to 2 to 3 weeks of glucocorticoid treatment, or do not achieve sustained response after discontinuation of glucocorticoid administration. Based upon current scientific knowledge, it is unclear whether splenectomy predisposes patients to AIDS.
In addition to prednisolone therapy and splenectomy, certain cytotoxic agents, e.g., vincristine, and azidothimidine (AZT, zidovudine) also show promise in treating HIV-induced ITP; however, the results are preliminary.
It will be appreciated from the foregoing that one way to treat thrombocytopenia would be to obtain an agent capable of accelerating the differentiation and maturation of megakaryocytes or precursors thereof into the platelet-producing form. Considerable efforts have been expended on identifying such an agent, commonly referred to as “thrombopoietin” (TPO). Other names for TPO commonly found in the literature include; thrombocytopoiesis stimulating factor (TSF), megakaryocyte colony-stimulating factor (MK-CSF), megakaryocyte-stimulating factor and megakaryocyte potentiator. TPO activity was observed as early as 1959 (Rak et al., Med. Exp., 1:125) and attempts to characterize and purify this agent have continued to the present day. While reports of partial purification of TPO-active polypeptides exist (see, for example, Tayrien et al., J. Biol. Chem., 262:3262 [1987] and Hoffman et al., J. Clin. Invest. 75:1174 [1985]), others have postulated that TPO is not a discrete entity in its own right but rather is simply the polyfunctional manifestation of a known hormone (IL-3, Sparrow et al., Prog. Clin. Biol. Res., 215:123 [1986]). Regardless of its form or origin, a molecule possessing thrombopoietic activity would be of significant therapeutic value. Although no protein has been unambiguously identified as TPO, considerable interest surrounds the recent discovery that mpl, a putative cytokine receptor, may transduce a thrombopoietic signal.
V. Mpl is a Cytokine Receptor
It is believed that the proliferation and maturation of hematopoietic cells is tightly regulated by factors that positively or negatively modulate pluripotential stem cell proliferation and multilineage differentiation. These effects are mediated through the high-affinity binding of extracellular protein factors to specific cell surface receptors. These cell surface receptors share considerable homology and are generally classified as members of the cytokine receptor superfamily. Members of the superfamily include receptors for: IL-2 (beta and gamma chains) (Hatakeyama et al., Science, 244:551-556 [1989]; Takeshita et al., Science, 257:379-382 [1991]), IL-3 (Itoh et al., Science, 247:324-328 [1990]; Gorman et al., Proc. Natl. Acad. Sci. USA, 87:5459-5463 [1990]; Kitamura et al., Cell, 66:1165-1174 [1991a]; Kitamura et al., Proc. Natl. Acad. Sci. USA, 88:5082-5086 [1991b]), IL-4 (Mosley et al., Cell, 59:335-348 [1989], IL-5 (Takaki et al., EMBO J., 9:4367-4374 [1990]; Tavernier et al., Cell, 66:1175-1184 [1991]), IL-6 (Yamasaki et al., Science, 241:825-828 [1988]; Hibi et al., Cell, 63:1149-1157 [1990]), IL-7 (Goodwin et al., Cell, 60:941-951 [1990]), IL-9 (Renault et al., Proc. Natl. Acad. Sci. USA, 89:5690-5694 [1992]), granulocyte-macrophage colony-stimulating factor (GM-CSF) (Gearing et al., EMBO J., 8:3667-3676 [1991]; Hayashida et al., Proc. Natl. Acad. Sci. USA, 244:9655-9659 [1990]), granulocyte colony-stimulating factor (G-CSF) (Fukunaga et al., Cell, 61:341-350 [1990a]; Fukunaga et al., Proc. Natl. Acad. Sci. USA, 87:8702-8706 [1990b]; Larsen et al., J. Exp. Med., 172:1559-1570 [1990]), EPO (D'Andrea et al., Cell, 57:277-285 [1989]; Jones et al., Blood, 76:31-35 [1990]), Leukemia inhibitory factor (LIF) (Gearing et al., EMBO J., 10:2839-2848 [1991]), oncostatin M (OSM) (Rose et al., Proc. Natl. Acad. Sci. USA, 88:8641-8645 [1991]) and also receptors for prolactin (Boutin et al., Proc. Natl. Acad. Sci. USA, 88:7744-7748 [1988]; Edery et al., Proc. Natl. Acad. Sci. USA, 86:2112-2116 [1989]), growth hormone (GH) (Leung et al., Nature, 330:537-543 [1987]) and ciliary neurotrophic factor (CNTF) (Davis et al., Science, 253:59-63 [1991].
Members of the cytokine receptor superfamily may be grouped into three functional categories (for review see Nicola et al., Cell, 67:1-4 [1991]). The first class comprises single chain receptors, such as erythropoietin receptor (EPO-R) or granulocyte colony stimulating factor receptor (G-CSF-R), which bind ligand with high affinity via the extracellular domain and also generate an intracellular signal. A second class of receptors, so called α-subunits, includes interleukin-6 receptor (IL6-R), granulocyte-macrophage colony stimulating factor receptor (GM-CSF-R), interleukin-3 receptor (IL3-Rα) and other members of the cytokine receptor superfamily. These α-subunits bind ligand with low affinity but cannot transduce an intracellular signal. A high affinity receptor capable of signaling is generated by a heterodimer between an α-subunit and a member of a third class of cytokine receptors, termed β-subunits, e.g., βC, the common β-subunit for the three α-subunits IL3-Rα and GM-CSF-R.
Evidence that mpl is a member of the cytokine receptor superfamily comes from sequence homology (Gearing, D. P., EMBO J., 8:3667-3676 [1989]; Bazan, J. F., Proc. Natl. Acad. Sci. USA, 87: 6934-6938 [1990]; Davis S., et al., Science, 253:59-63 [1991] and Vigon et al., Proc. Natl. Acad. Sci. USA, 89:5640-5644 [1992]) and its ability to transduce proliferative signals.
Deduced protein sequence from molecular cloning of murine c-mpl reveals this protein is homologous to other cytokine receptors. The extracellular domain contains 465 amino acid residues and is composed of two subdomains each with four highly conserved cysteines and a particular motif in the N-terminal subdomain and in the C-terminal subdomain. The ligand-binding extracellular domains are predicted to have similar double β-barrel fold structural geometries. This duplicated extracellular domain is highly homologous to the signal transducing chain common to IL-3, IL-5 and GM-CSF receptors as well as the low-affinity binding domain of LIF (Vigon et al., Oncogene, 8:2607-2615 [1993]). Thus mpl may belong to the low affinity ligand binding class of cytokine receptors.
The extracellular domain is followed by a 22 residue transmembrane domain and a 121 residue cytoplasmic domain rich in serine and proline. The cytoplasmic domain contains no consensus protein kinase or phosphatase motif associated with signal transduction.
A comparison of murine mpl and mature human mpl P, reveals these two proteins show 81% sequence identity. More specifically, the N-terminus and C-terminus extracellular subdomains share 75% and 80% sequence identity respectively. The most conserved mpl region is the cytoplasmic domain showing 91% amino acid identity, with a sequence of 37 residues near the transmembrane domain being identical in both species. Accordingly, mpl is reported to be one of the most conserved members of the cytokine receptor superfamily (Vigon supra).
Evidence that mpl is a functional receptor capable of transducing a proliferative signal comes from construction of chimeric receptors containing an extracellular domain from a cytokine receptor having high affinity for a known cytokine with the mpl cytoplasmic domain. Since no known ligand for mpl has been reported, it was necessary to construct the chimeric high affinity ligand binding extracellular domain from a class one cytokine receptor such as IL-4R or G-CSFR. Vigon et al., supra fused the extracellular domain of G-CSFR with both the transmembrane and cytoplasmic domain of c-mpl. An IL-3 dependent cell line, BAF/B03 was transfected with the G-CSFR/mpl chimera along with a full length G-CSFR control. Cells transfected with the chimera grew equally well in the presence of cytokine IL-3 or G-CSF. Similarly, cells transfected with G-CSFR also grew well in either IL-3 or G-CSF. All cells died in the absence of growth factors. A similar experiment was conducted by Skoda et al., EMBO J., 12(7):2645-2653 (1993] in which both the extracellular and transmembrane domains of human IL-4 receptor (hIL-4-R) were fused to the murine mpl cytoplasmic domain, and transfected into a murine IL-3 dependent Ba/F3 cell line. Ba/F3 cells transfected with wildtype hIL-4-R proliferated normally in the presence of either of the species specific IL-4 or IL-3. Ba/F3 cells transfected with hIL-4R/mpl proliferated normally in the presence of hIL-4 (in the presence or absence of IL-3) demonstrating that in Ba/F3 cells the mpl cytoplasmic domain contains all the elements necessary to transduce a proliferative signal.
These chimeric experiments demonstrate the proliferation signaling capability of the mpl extracellular domain but are silent regarding whether the mpl extracellular domain can bind a ligand. These results are consistent with at least two possibilities, namely, mpl is a single chain (class one) receptor like EPO-R or G-CSFR or it is a signal transducing β-subunit (class three) requiring an α-subunit like IL-3 (Skoda et al. supra).
VI. Mpl Ligand Stimulates Megakaryocytopoiesis
As described above, it has been suggested that serum contains a unique factor, sometimes referred to as thrombopoietin, that acts synergistically with various other cytokines to promote growth and maturation of megakaryocytes. No such natural factor has ever been isolated from serum or any other source even though considerable effort has been expended by numerous groups. Even though it is not known whether mpl is capable of directly binding a megakaryocyte stimulating factor, recent experiments demonstrate that mpl is involved in proliferative signal transduction from a factor or factors found in the serum of patients with aplastic bone marrow (Methia et al., Blood, 82(5): 1395-1401 [1993]).
Evidence that a unique serum colony-forming factor distinct from IL-1α, IL-3, IL-4, IL-6, IL-11, SCF, EPO, G-CSF, and GM-CSF transduces a proliferative signal through mpl comes from examination of the distribution of c-mpl expression in primitive and committed hematopoietic cell lines and from mpl antisense studies in one of these cell lines.
Using reverse transcriptase (RT)-PCR in immuno-purified human hematopoietic cells, Methia et al., supra demonstrated that strong mpl mRNA messages were only found in CD34+ purified cells, megakaryocytes and platelets. CD34+ cells purified from bone marrow (BM) represents about 1% of all BM cells and are enriched in primitive and committed progenitors of all lineages (e.g., erythroid, granulomacrophage, and megakaryocytic).
Mpl antisense oligodeoxynucleotides were shown to suppress megakaryocytic colony formation from the pluripotent CD34+ cells cultured in serum from patients with aplastic marrow (a rich source of megakaryocyte colony-stimulating activity [MK-CSA]). These same antisense oligodeoxynucleotides had no effect on erythroid or granulomacrophage colony formation.
Whether mpl directly binds a ligand and whether the serum factor shown to cause megakaryocytopoiesis acts through mpl is still unknown. It has been suggested, however, that if mpl does directly bind a ligand, its amino acid sequence is likely to be highly conserved and have species cross-reactivity owing to the considerable sequence identity between human and murine mpl extracellular domains (Vigon et al., supra [1993]).
In view of the foregoing, it will be appreciated there is a current and continuing need in the art to isolate and identify molecules capable of stimulating proliferation, differentiation and maturation of hematopoietic cells, especially megakaryocytes or their predecessors for therapeutic use in the treatment of thrombocytopenia. It is believed such a molecule is a mpl ligand and thus there exists a further need to isolate such ligand(s) to evaluate their role(s) in cell growth and differentiation.
Accordingly, it is an object of this invention to obtain a pharmaceutically pure molecule capable of stimulating proliferation, differentiation and/or maturation of megakaryocytes into the mature platelet-producing form.
It is another object to provide the molecule in a form for therapeutic use in the treatment of a hematopoietic disorder, especially thrombocytopenia.
It is a further object of the present invention to isolate, purify and specifically identify protein ligands capable of binding in vivo a cytokine superfamily receptor known as mpl and to transduce a proliferative signal.
It is still another object to provide nucleic acid molecules encoding such protein ligands and to use these nucleic acid molecules to produce mpl binding ligands in recombinant cell culture for diagnostic and therapeutic use.
It is yet another object to provide derivatives and modified forms of the protein ligands including amino acid sequence variants, variant glycoprotein forms and covalent derivatives thereof.
It is an additional object to provide fusion polypeptide forms combining a mpl ligand and a heterologous protein and covalent derivatives thereof.
It is still an additional object to provide variant polypeptide forms combining a mpl ligand with amino acid additions and substitutions from the EPO sequence to produce a protein capable of regulating proliferation and growth of both platelets and red blood cell progenitors.
It is yet an additional object to prepare immunogens for raising antibodies against mpl ligands or fusion forms thereof, as well as to obtain antibodies capable of binding such ligands.
These and other objects of the invention will be apparent to the ordinary artisan upon consideration of the specification as a whole.